The present invention relates to a method for producing fatty acid alkyl esters, involving esterifying a material containing free fatty acids with an alcohol and an inorganic acid catalyst to form a product containing fatty acid alkyl esters, wherein (i) the material contains at least about 40% FFA and is produced by reacting a feedstock with steam and sulfuric acid at a pH of about 1-about 2 or (ii) the material contains at least about 80% FFA and is produced by reacting a feedstock with steam and alkali at a pH of about 10-about 14 and further reacting the feedstock with steam and sulfuric acid at a pH of about 1-about 2. The present invention also relates to a method for producing a lipid rich composition containing at least about 80% free fatty acids, the method involving reacting a feedstock with steam and alkali at a pH of about 10-about 14 and further reacting the feedstock with steam and sulfuric acid at a pH of about 1-about 2. Furthermore, the present invention concerns a lipid rich composition containing at least about 80% free fatty acids.
Over the past three decades interest in the reduction of air pollution, and in the development of domestic energy sources, has triggered research in many countries on the development of non-petroleum fuels for internal combustion engines. For compression ignition (diesel) engines, it has been shown that the simple alcohol esters of fatty acids (biodiesel) are acceptable alternative diesel fuels. Biodiesel has a higher oxygen content than petroleum diesel, and therefore reduces emissions of particulate matter, hydrocarbons, and carbon monoxide, while also reducing sulfur emissions due to a low sulfur content (Sheehan, J., et al., Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in an Urban Bus, National Renewable Energy Laboratory, Report NREL/SR-580-24089, Golden, Colo. (1998); Graboski, M. S., and R. L. McCormick, Prog. Energy Combust. Sci., 24:125-164 (1998)). Since it is made from agricultural materials, which are produced via photosynthetic carbon fixation (e.g., by plants and by animals that consume plants), the combustion of biodiesel does not contribute to net atmospheric carbon levels.
Initial efforts at the production, testing, and use of biodiesel employed refined edible vegetable oils and animal fats (e.g., beef tallow) as feedstocks for fuel synthesis (Krawczyk, T., INFORM, 7: 800-815 (1996); Peterson, C. L., et al., Applied Engineering in Agriculture, 13: 71-79 (1997); Holmberg, W. C., and J. E. Peeples, Biodiesel: A Technology, Performance, and Regulatory Overview, National Soy Diesel Development Board, Jefferson City, Mo. (1994)). Simple alkali-catalyzed transesterification technology (Freedman, B., et al., J. Am. Oil Chem. Soc., 61(10): 1638-1643 (1984)) is efficient at esterifying the acylglycerol-linked fatty acids of such feedstocks and is employed in making these fuels. More recently, methods have been developed to produce fatty acid methyl esters (FAME) from cheaper, less highly refined lipid feedstocks such as spent restaurant grease (Mittelbach, M., and P. Tritthart, J. Am Oil Chem. Soc., 65(7):1185-1187 (1988); Graboski, M. S., et al., The Effect of Biodiesel Composition on Engine Emissions from a DDC Series 60 Diesel Engine, Final Report to USDOE/National Renewable Energy Laboratory, Contract No. ACG-8-17106-02 (2000); Haas, M. J., et al., Enzymatic Approaches to the Production of Biodiesel Fuels, in Kuo, T. M. and Gardner, H. W. (Eds.), Lipid Biotechnology, Marcel Dekker, Inc., New York, (2002), pp. 587-598). In addition to acylglycerols, less highly refined lipid feedstocks can contain substantial levels of free fatty acids (FFA) and other nonglyceride materials. Biodiesel synthesis from these feedstocks can be accomplished by conventional alkaline catalysis, which then requires an excess of alkali since the FFA (which are not esterified by this method) are converted to their alkali salts. These alkali salts can cause difficulties during product washing due to their ready action as emulsifiers. Ultimately, the alkali salts are removed and discarded. This approach thus involves a loss of potential product, increases catalyst expenses, and can entail a disposal cost. Alternatively, multi-step processes involving acid-catalyzed esterification of the free fatty acids and alkali-catalyzed transesterification of glyceride-linked fatty acids can be employed to achieve more efficient conversion of heterogenous feedstocks (Canakci, M., and J. Van Gerpen, Biodiesel Production from Oils and Fats with High Free Fatty Acids, Abstracts of the 92nd American Oil Chemists' Society Annual Meeting & Expo, p. S74 (2001); U.S. Pat. Nos. 2,383,601; 2,494,366; 4,695,411; 4,698,186; 4,164,506). However, these methods can require multiple acid-catalyzed esterification steps to reduce the concentration of free fatty acids to acceptably low levels.
In addition to waste greases, other lipid-rich materials of relatively low value are potential sources of biodiesel. Among these is soapstock (SS), a coproduct of the refining of edible vegetable oils (e.g., soybean). Soapstock is an alkaline emulsion composed largely of water, acylglycerols, phosphoacylglycerols, and FFA. It is generated at a rate of about 6% of the input of unrefined oil entering a refining operation, amounting to approximately 100 million lbs annually in the United States. Although there are some industrial uses for SS, demand fluctuates and the economic return to the producer is not high, leading to interest in the development of new uses for this material.
We previously reported methods for the production of fatty acid methyl esters (FAME) from soybean SS (Haas, M. J., et al., J. Am. Oil Chem. Soc., 77:373-379 (2000)) and established that the performance and emissions properties of the resulting fuel were comparable to those of commercial biodiesel from refined soybean oil (Haas, M. J., et al., Energy & Fuels, 15(5):1207-1212 (2001)). This method for FAME synthesis employs sequential alkali-catalyzed saponification, water removal, and acid-catalyzed esterification to produce esters from both the lipid-linked and the free fatty acids of SS. The method achieves the efficient production of high purity biodiesel; however, it suffers from the fact that substantial amounts of solid sodium sulfate are generated as a byproduct. Disposal of this waste material could be cumbersome and expensive. Therefore there is a need for further development of routes for the production of fatty acid alkyl esters (e.g., FAME) from SS and similar complex lipid mixtures.